Probably to my 14 year old self, stop being a whiny little bitch, lose that fat before the age of 17 and put some effort into school.
I dont like my past self, I think I would be cold and sarcastic towards him if we had a conversation.

I would say "There will be very hard times in the upcoming future but remember your not alone, don't isolate yourself. Your loved and You will get happy again and you will have beautiful experiences or I wouldn't be here to tell you that. Keep hanging in there."